Erbium-doped waveguide amplifiers (EDWAs) have many advantages over erbium-doped fiber amplifiers (EDFAs) due to their potential for reducing the costs and sizes, and integration with other waveguide devices. Different fabrication techniques have been used to make these Er-doped planar waveguide devices, such as ion-exchange, radio-frequency sputtering, sol-gel, plasma-enhanced chemical vapor deposition, and others. As expected, different techniques lead to different formations and qualities of the waveguides. The ion-exchange technique is attractive for making graded-index surface and buried waveguide devices because of its simplicity and low cost. Currently, the main difficulty in EDWAs, as compared to EDFAs, is that a sufficiently high gain must be obtained in a much shorter device length. This implies using about two orders of magnitude higher Er3+ ions concentration, with correspondingly higher risk of cluster formation and stronger ion-ion interactions. Important issues are to optimize the glass material composition (which often implies co-doping with Yb3+ ions) and the waveguide fabrication process. Many studies have been carried out on optical glasses activated by rare-earth ions. Phosphate glasses are known to be suitable rare-earth host materials because of their spectroscopic characteristics, including a large emission cross section and a weak interaction among rare-earth ions. It is possible to achieve a high gain within a short waveguide length. These glasses have been used to produce ion-exchanged waveguide amplifiers with more than 3 dB/cm gain coefficients.
Optical waveguides have been fabricated in Na+-based glasses using either a K+—Na+ or an Ag+—Na+ ion-exchange method. The K+—Na+ ion-exchange process has several advantages compared to the Ag+—Na+ ion-exchange process: lower losses, increased stability, and less expensive. Buried waveguides have attracted much attention because of their low surface scattering losses and symmetric refractive index profiles, and the mode profiles match well with those of optical fibers. Various experimental approaches have been used to achieve this goal. In general, field-assisted ion-exchange (FAIE) is often used to fabricate buried waveguides including one-step electromigration technique and two-step ion-exchange. Examples of such prior art can be found in U.S. Pat. No. 5,318,614, U.S. Pat. No. 5,269,888, U.S. Pat. No. 4,842,629 and EP0269996A.
Field-assisted ion exchange is based on the accelerated migration of incoming ions by applying an electric field across a glass substrate between its plane surfaces. However, such methods involve using molten salt as the electrode, are complicated and require an expensive special sample holder and sealed materials. One known dry-technique that does not use a molten salt is a silver film ion-exchange process (U.S. Pat. No. 5,491,708, U.S. Pat. No. 5,160,523).